1. Field of the Invention
The present invention relates to a silver-metal oxide composite material and process for producing the same, and in particular to a silver-metal oxide composite material suited to electrical contact materials and electrode materials for electric welding and a process for producing it.
2. Description of Prior Art
Silver-metal oxide composite materials prepared by adding a metal oxide such as a tin oxide to silver have a markedly improved strength and therefore are used as an electrical contact material for relays, switches, breakers, and the like for alternating current and direct current, particularly suitably used as electrical switching contact materials for medium load purposes.
Silver-metal oxide composite materials have been heretofore produced by the methods in which a silver alloy containing one or more other metals to be oxidized is internally oxidized, or a silver powder and a powder of an oxide of other metals are sintered by power metallurgy.
According to the above internal oxidation method, a silver-other metals solid solution alloy is heated below its melting point under an increased partial pressure of oxygen so that oxygen may be diffused into the alloy, thereby the other metals which have a relatively high affinity for oxygen being precipitated as fine particles of oxides in a silver matrix. This method, however, has the disadvantages that the oxide content achieved in the composite material produced is limited to not more than about 4% by weight in terms elemental metal, and that the diffusion rate of oxygen into the solid solution alloy is so low that production of the composite material needs much time. To increase the oxide content above about 4% in terms of elemental metal or to increase the diffusion rate of oxygen, an element capable of promoting oxidation such as In and Bi is added prior to internal oxidation. Nevertheless, internal oxidation of an alloy with a thickness of, e.g., 2 mm takes about one month.
Moreover, according to internal oxidation, the amount of oxygen diffusing into a solid solution alloy decreases in adverse proportion to the square of the thickness of the layer from the surface which has been already oxidized, so that it is inevitable that oxide particles close to the surface become coarse, whereas an alloy phase containing a small amount of fine oxide particles forms in the core. Consequently, the silver-metal oxide composite material produced is non-uniform in the distribution of the oxide particles as well as in the size thereof. The particle size decreases with the depth. Since the oxide particles are non-uniform in size and segregate as described above, improvement in strength of the composite material obtained is limited; hence further improvement has been required.
In the production of a silver-metal oxide composite material according to powder metallurgy, a powder of an oxide of Sn, Cd, Zn or the like with good refractory properties and a silver powder are sintered at a temperature at which silver is solid. Therefore, strong binding is not achieved between the silver phase and the oxide particles; there remains fine spaces therebetween. Further defects existing in the crystal structure of the starting oxide are not repaired. Consequently, the sintered product obtained has a poor mechanical strength, particularly at a high temperature, which cannot be improved even by post-treatment such as hot extrusion or forging. To improve the silver-metal oxide composite material produced by powder metallurgy, the addition of W, Mo or the like that forms lower oxides is attempted, but it increases contact resistance and makes the resulting composite material susceptible to deposition where the material is used as an electrical contact material. The addition of MnO, CaO, ZrO or the like for improvement may be proposed, but it impairs sintering properties and therefore results in a lowering of the mechanical strength of the sintered products obtained.